cg.cgcookie.com/2008/11/29/retopology-tools-with-blender

This tutorial explains the retopology tool using the standard CTRL+Left Mouse Button to add vertices. This works well enough for the subject matter covered in the tutorial.

You may however want to use the paint function to draw the rough mesh rather than the method shown in the tutorial.

To do this click the paint button that is visible directly below the Retopo in Step 03. This will put the tool into sketch mode then you simply draw the base mesh over the existing model.

The mesh is created and snapped to the original model automatically when you return to normal editing mode. You can then edit the resulting mesh as you would any other mesh object.

Are you planning to make a back plate also? If so I personally would create a basic lowpoly sleeveless "shirt", instead of using shrinkwrap or retopo, fit it to the figure then split that into separate front and back  base objects to further manipulate.

Are you starting to get the idea that there are multiple ways to handle just about any modeling project?:lol:

(Learn the different ways, then you have the flexibility to use whichever method works best for a given situation.)